There are many musical instruments which have an electrical pick-up that has a relatively high impedance (e.g., about 250K ohms or above) which outputs a signal that must be coupled to an amplifier which may be some distance away. In order to maintain fidelity, sustain, and avoid noise, the output impedance of the musical instrument must be properly matched to the input impedance of the amplifier. While proper termination has long been a factor in professional audio implementations, it has not been previously considered in the proper interface of the pick-up based instruments.
The object of the present invention is to provide a proper termination of the instrument pick-up and, at the same time, provide a proper source impedance to the amplifier. While the present invention does not amplify the signal, it does provide noticeable improvements in matching which results in a wider and a more even frequency response with the lower noise and with an apparent increase in volume. Sustain characteristics of the instrument are also improved. Thus, the present invention prevents deterioration of the instrument signal due to loss of fidelity and build-up in noise, as compared to normal instrument interface techniques.
According to the invention, an impedance matching cable system for electronically coupling a musical instrument to an amplifier includes a field effect transistor (FET) having gate, source and drain electrodes and a bipolar direct current voltage supply having a pair of end voltage point connections and a center point voltage connection. A current control device connects the center voltage connection point to the common conductor of the musical instrument and the signal conductor of the musical instrument is connected to the GATE electrode of the field effect transistor, and a DC isolating capacitor couples an AC signal voltage from the drain electrode to the amplifier. A high resistor is connected between the GATE electrode and the common conductor.
In a preferred embodiment, a male telephone-type plug connector has a tubular housing and the field effect transistor is mounted within the tubular housing and epoxyed or sealed-in. In a further preferred embodiment, the bipolar direct current supply and its voltage connection points are connected to three terminal XLR female connectors and the source and drain electrodes and common conductor are connected to a corresponding three-terminal XLR male connector. To provide greater distances, a shielded cable with a pair of signal conductors and a shield conductor has a corresponding XLR female plug at one end thereof and an XLR male plug at the opposite end which are adapted to connect the bipolar battery supply system to the FET transistor.